[CMake] Improve GetHostTriple for Windows/ARM configurations

In MSVC mode, CMake provides CMAKE_C_COMPILER_ARCHITECTURE_ID,
which should be accurate for what the compiler produces. Keep
the old CMAKE_C_SIZEOF_VOID_P based fallback in case this
doesn't provide matches.

In GCC/MinGW mode, CMake doesn't provide that. As we're trying to
guess the default host architecture, use CMAKE_HOST_SYSTEM_PROCESSOR
in combination with the existing CMAKE_C_SIZEOF_VOID_P check to
estimate whether it's x86/x86_64/arm/aarch64.

Differential Revision: https://reviews.llvm.org/D107626